(1) Field of the Invention
The present invention relates to a light source device for use in a projection-type display apparatus such as a liquid crystal projector or the like, and more particularly to an illuminating apparatus and projection-type display apparatus which employ such a light source device.
(2) Description of the Prior Art
Light source devices for use in projection-type display apparatus have a light source comprising a discharge lamp such as a xenon lamp which is of high luminance and high emission efficiency, a high-pressure mercury lamp, or a metal halide lamp, and a reflecting mirror for reflecting rays of light radiated from the light source into a parallel beam of light. FIG. 1 of accompanying drawings is a fragmentary cross-sectional view of a conventional light source device.
As shown in FIG. 1, the conventional light source device comprises discharge lamp 100 having a filling material capable of maintaining discharged light emission and a pair of electrodes 102a, 102b which are sealed in tubular transparent bulb 103 of quartz or the like that is of a substantially uniform thickness, and reflecting mirror 101 having paraboloid 101a as a reflecting surface whose focal point is positioned at light emission center 100a of discharge lamp 100. Rays of light radiated from light emission center 100a of discharge lamp 100 are reflected by paraboloid 101a of reflecting mirror 101, and travel substantially parallel to the axis (optical axis f) of paraboloid 101a. 
The effective range in which the rays of light that are emitted from light emission center 100a of discharge lamp 100 can be used is defined between a ray xe2x80x9caxe2x80x9d of light (inclined at angle xcex81 to optical axis f) falling on paraboloid 101a and ray xe2x80x9cbxe2x80x9d of light (inclined at angle xcex82 to optical axis f) falling on paraboloid 101a. FIG. 2 of the accompanying drawings shows a discharge lamp which is designed to increase the above effective range for an increased effective ray availability ratio (the ratio of rays of reflected light that can be used to all the rays of light emitted from the discharge lamp. The discharge lamp shown in FIG. 2 is basically of the same structure as the discharge lamp shown in FIG. 1 except that transparent bulb 103 has a lens structure. Specifically, transparent bulb 103 shown in FIG. 2 has wall thickness D1 at its center which is greater than wall thickness D2 at a sealed end thereof, enabling transparent bulb 103 to produce a lens effect. FIG. 2 also shows the rays xe2x80x9caxe2x80x9d, xe2x80x9cbxe2x80x9d of light shown in FIG. 1 which are indicated by solid lines. The lens effect of transparent bulb 103 causes rays xe2x80x9cdxe2x80x9d, xe2x80x9ccxe2x80x9d of light emitted outside of the range between the rays xe2x80x9caxe2x80x9d, xe2x80x9cbxe2x80x9d of light in the discharge space of the transparent bulb 103 to fall on paraboloid 101a of reflecting mirror 101. Therefore, the effective ray availability ratio of the light source shown in FIG. 2 is better than the effective ray availability ratio of the light source shown in FIG. 1.
However, since the transparent bulb having the lens effect refracts the rays of light emitted therefrom, the focal point of paraboloid 101a is shifted out of alignment with light emission center 100a, failing to produce parallel rays of light reflected from reflecting mirror 101. Details of such a phenomenon are illustrated in FIG. 3 of the accompanying drawings. As shown in FIG. 3, paraboloid 101a is divided into regions I, II by a boundary where ray xe2x80x9cixe2x80x9d of light emitted from light emission center 100a and passing, unrefracted, through transparent bulb 103 is applied to paraboloid 101a. Region I extends from the boundary toward the closed end or crest of reflecting mirror 101, whereas region II extends from the boundary toward the open end of reflecting mirror 101. Ray xe2x80x9cixe2x80x9d of light is reflected by paraboloid 101a and travels parallel to optical axis f. However, rays xe2x80x9chxe2x80x9d, xe2x80x9cgxe2x80x9d of light reflected by region I and rays xe2x80x9ckxe2x80x9d, xe2x80x9cjxe2x80x9d of light reflected by region II do not travel parallel to optical axis f. That is, rays xe2x80x9chxe2x80x9d, xe2x80x9cgxe2x80x9d of light reflected by region I spread at respective angles away from optical axis f, and rays xe2x80x9ckxe2x80x9d, xe2x80x9cjxe2x80x9d of light reflected by region II are intersected at respective angles toward optical axis f.
Therefore, the conventional attempt to increase the effective ray availability ratio of the light source with the lens structure of the transparent bulb has reduced the parallelism of the rays of light reflected by the reflecting mirror with the optical axis f.
Japanese laid-open patent publication No. 2000-138005 has proposed a transparent bulb that can easily be controlled in shape at the time it is fabricated, the transparent bulb having an outer wall shaped to produce parallel rays of light. However, changing the shape of the outer surface of the transparent bulb as a condensing lens poses a limitation on the correction of parallelism of the reflected rays of light.
A first object of the present invention is to provide a light source device which is of high luminance and is capable of emitting rays of light in high parallelism.
A second object of the present invention is to provide an illuminating apparatus which is of high luminance and is capable of emitting rays of light in high parallelism, using such a light source device.
A third object of the present invention is to provide a projection-type display apparatus which is of high luminance and is capable of emitting rays of light in high parallelism, using such a light source device.
To achieve the first object, a light source device according to the present invention includes a discharge lamp having a tubular transparent bulb with sealed opposite ends for generating discharged light emission near a light emission center thereof, and a reflecting mirror having a reflecting surface which comprises a curved surface approximating a paraboloid whose focal point is positioned at the light emission center of the transparent bulb, the transparent bulb having a lens structure whose wall thickness is greater at the light emission center thereof than at the sealed ends thereof, the reflecting surface being divided by a boundary where a ray of light emitted from the light emission center and passing, unrefracted, through the transparent bulb is applied to the reflecting surface, into a first curved surface extending from the boundary toward an open end of the reflecting mirror and a second curved surface extending from the boundary toward a closed end of the reflecting mirror, which is remote from the open end, the second curved surface being shaped such that the angle of incidence of a ray of light emitted from the light emission center and applied to the second curved surface is progressively smaller toward the closed end, and changes of the angle of incidence are greater than changes of the angle of incidence of a ray of light on the paraboloid.
The first curved surface may be arranged such that the angle of incidence of a ray of light emitted from the light emission center and applied to the first curved surface is progressively greater in a direction away from the boundary, and changes of the angle of incidence are greater than changes of the angle of incidence of a ray of light on the paraboloid.
To achieve the second object, an illuminating apparatus according to the present invention includes the above light source device and a condensing optical system for converging rays of light emitted from the light source device to produce uniform illuminating light.
To achieve the third object, a projection-type display apparatus according to the present invention includes the above light source device and projection image generating means for partly passing light emitted from the light source device to generate a projection image.
The present invention offers the following advantages:
The angle of incidence of a ray of light refracted by and passing through the transparent bulb, and applied to the paraboloid whose focal point is positioned at the light emission center of the transparent bulb is greater near the closed end of the reflecting mirror and smaller near the open end thereof than the angle of incidence of a ray of light passing, unrefracted, through the transparent bulb. With the paraboloid used as a reflecting surface, as is conventional, the angle of incidence is varied due to the refraction of rays of light, lowering the parallelism of the rays of light with the optical axis of the reflecting mirror.
According to the present invention, the second curved surface closer to the closed end of the reflecting mirror is shaped such that the angle of incidence of a ray of light thereon is progressively smaller in a direction away from the boundary, and changes of the angle of incidence are greater than changes of the angle of incidence of a ray of light on the paraboloid. Therefore, any variations of the angle of incidence on the second curved surface due to the refraction are compensated for, and rays of light refracted by and passing through the transparent bulb are reflected by the second curved surface to travel parallel to the optical axis, i.e., the axis of the paraboloid.
Furthermore, the first curved surface closer to the open end of the reflecting mirror is shaped such that the angle of incidence of a ray of light thereon is progressively greater in a direction away from the boundary, and changes of the angle of incidence are greater than changes of the angle of incidence of a ray of light on the paraboloid. Therefore, as with the second curved surface, any variations of the angle of incidence on the first curved surface due to the refraction are compensated for, and rays of light refracted by and passing through the transparent bulb are reflected by the first curved surface to travel parallel to the optical axis, i.e., the axis of the paraboloid.
According to the present invention, the light source device is free of the conventional problem of poor parallelism of rays of light reflected by the reflecting mirror with the optical axis thereof as a result of the lens structure employed by the transparent bulb for an increased effective ray availability ratio.
The above and other objects, features, and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention.